RF Link Transmitter - 434MHz - only one in the same location?

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hugoxp1

Member
Hi,

I am thinking about buying this transmitter: http://www.sparkfun.com/commerce/product_info.php?products_id=8946 and put it to work with the PICAXE 20X2.

I will need 3 transmitters to work at same time sending diferent data (i will use 3 transmitters with 3 picaxes) and only one receiver: http://www.sparkfun.com/commerce/product_info.php?products_id=8950 (attached with one more picaxe).

I read on the above website: "Only one 434MHz transmitter will work within the same location."

my question is: I can not have more than one transmitter sending data at same time? why not?

thank you
 
hippy

hippy

Ex-Staff (retired)
Imagine three people were going to communicate with you at the same time by using Morse Code. If they all transmitted together you'd not have a clue what the message from any of them was, what you hear would all be a complete mess.

It's the same with multiple transmitters on the same frequencies.
 
H

hugoxp1

Member
hippy said:
Imagine three people were going to communicate with you at the same time by using Morse Code. If they all transmitted together you'd not have a clue what the message from any of them was, what you hear would all be a complete mess.

It's the same with multiple transmitters on the same frequencies.
Click to expand...
But, if i send always a fixed value:

transmitter/Picaxe 1:
Code: 
serout 7,N2400,(1,b1,b2,b3)

transmitter/Picaxe 2:
Code: 
serout 7,N2400,(2,b1,b2,b3)
transmitter/Picaxe 3:
Code: 
serout 7,N2400,(3,b1,b2,b3)

i can always know where the data came from, right?

(I was more asking if one transmitter does not "eliminate" the frequency of another transmitter)

thank you
 
C

Chavaquiah

Senior Member
Depending on how much information you need to transmit and how important it is to have it received in a timely manner, you MIGHT get away with using more than one transmitter.

Obviously, as hippy explained (quite well), if two or more transmitters try to send at the same time, transmissions will be garbled and unreadable. However, if all you need is to have a couple of bytes transmitted per second and it is alright to miss a few messages, you can program each transmitter to repeat each message several times and at differing intervals. Odds are some of the messages will get through.

For example, I’m building a positioning system for robots. I have several (3 to 6 per room) IR emitters. Each emitter just sends it’s ID. The robots have an array of sensors that pick the direction and the ID of the signals they receive. When two emitters try to send simultaneously, the signals gets mixed/garbled and must be ignored. However, since they all keep repeating the same signal a few times per second, eventually they get to be “heard” properly, so the system works… sort of.

But, just in case, start looking at ZigBees, Wi-Flies, Bluetoothes or stuff like that.
 
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MFB

Senior Member
Further to the above advice, it would help if the space between transmissions were varied randomly. A sum check, or some other form of error detection, would have to be implemented at the receiver to deal with the inevitable collisions.
 
hippy

hippy

Ex-Staff (retired)
There's also a little more complexity to it when two transmitters are powered on together. One signal from one transmitter can drown out another, even when not actually transmitting. It's not always the closest/strongest which drowns out the furthest/weakest.
 
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manuka

Senior Member
Hang on a few days perhaps, as Dr_A may be able to report tempting findings with some new SURE RMB-CM12111 GFSK units. You'll still have data collisions of course, BUT their features may allow some workarounds.
 

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premelec

premelec

Senior Member
If you use transceivers and pseudo random transmission times AND check the frequency for activity BEFORE starting a transmission from each unit then you have a higher chance of getting the message through... some transceivers are pretty cheap and have RSS outputs on them to easily determine if the frequency is busy... think of it in terms of two people trying to talk to each other - they must take turns...
 
lbenson

lbenson

Senior Member
I have used the Sparkfun 433mHz (and 315mHz) units and saw that note. It is, of course, too absolute. I have a half dozen transmitters sending temperatures (only when there is a 2 degree C change) and other data to a single receiver. I send each message 3 times and vary the pauses between. I'm sure there have been conflicts, but the system works well. If something were critical, I'd want to be able to send an acknowledgement and have the transmitter keep trying until the unit got it.
 
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moxhamj

New Member
As manuka says, I've been testing those new Sure modules. $29 a pair from the factory and they are brilliant. Programmable baud rates, 512 byte buffer and programmable channels. Plus TTL, RS232 and RS485. Not only can they send a few bytes, I managed to send the source code for an entire operating system.

Having said that, for the original problem of data clashes, I'd suspect the cheap and cheerful $2 modules would work fine. It is just a matter of making sure data clashes are infrequent and don't cause corrupted data. So keep the messages short and infrequent, and with different delays for each board. Also, a very simple checksum protocol and probably the easiest if you are sending only one byte is to send it twice, and if the two bytes are equal then the data is extremely unlikely to be invalid.
 
premelec

premelec

Senior Member
Hi Drac... could you give a link to the source of these units ? And I wonder over what distance did you send the entire source code for an operating system - and is this statement good for April 2nd as well? Thanks!
 
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moxhamj

New Member
It is after midday here..

http://cgi.ebay.com/2xWireless-RF-Transceiver-431-478MHz-GFSK-Data-Transfer_W0QQitemZ350334728051QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item519193b773 they are quite hard to find so hopefully this link will work. Manuka is getting them for lower prices.

Distance is just over a hundred metres through trees and height of rx and tx 2.5 metres above the ground. But I suspect it can go further as it was 100% reliable. Also that was at 9600 baud and as a general rule RF goes further at lower baud rates.

I could post a photo but it does not contain a picaxe in the photo so in the interests of forum etiquette I'll not post that photo. Picaxes are the sensors and the controllers in the network. Other chips do the routing.

The operating system source code is a customised version CP/M and it is capable of recompiling its own source. Code is Sbasic and I think I might be the only person in the world using sbasic, but it looks very similar to picaxe basic (and not at all like mbasic). No line numbers. 50k of code space and megabytes of drive space on an sd card. Runs about the same speed as picaxe basic. The extra code space is useful for things like xmodem for file transfers.

I'm working on packet code and very close to getting that working. Packets can be short or long and the short ones do fit in a picaxe so a picaxe can send data out into the network and, somewhere, another picaxe will get that packet.
 
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manuka

Senior Member
See the Sure Electronics site here for more details. For your viewing pleasure I've attached further pix.

Note-although these are new to Sure Electronics,they're sold elsewhere as the Appcon Technologies APC200A-43 etc. Diverse US/Can outlets handle them, at prices ~twice that of the ex. works (China) cost,while UK firm DROBOTICS sell the similar APC220-43 (at £33 a pair),but they're apparently out of stock.
 

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moxhamj

New Member
Oh darn. Dippy has worked it out.

manuka, well done with that find. 'sureelectronics1' seems $10 more expensive than 'sureelectronics'. In the comments section there is a comment about shipping, so probably less of a difference for a couple but more if you buy lots.

Oh, I forgot to mention you can also set the Tx power via software, so that could be quite handy for conserving power. Drop it to the minimum that works then add a little.
 
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manuka

Senior Member
Dippy (et.al.)

The many existing "cheap & cheerful" ASK/FSK 433 MHz products are often fine for simple wireless data use, but tend too broad, weak or "deaf" for demanding applications. As the 433 MHz ISM spectrum slice is ~1.7MHz wide (433.05 - 434.79 MHz), but limited to ~25 mW power,it became apparent to me that GFSK (Gaussian Freq. Shift Keying) types would be superior. GFSK shapes FSK pulses,and thereby offers narrow band TX & more sensitive RX. It's a spectral efficiency winner,but module prices have naturally been higher.

It's a fast moving field & Chinese GFSK offerings have recently been emerging, no doubt driven by home demand. Many Asian cities are too high density for 2.4GHz ZigBee etc, & the crowded 433 MHz ISM band is instead used for utility meter reading. Hands on (& ordering!) experiences are naturally worth promoting for others- I certainly don't bow to backhanders from suppliers, & treat the many new releases on their merits,pricing & "supply".

FWIW the Aust/NZ UHF CB 40 channel "PRS" system at ~477 MHz allocates 2 data channels (Ch.22/23) for "3 seconds an hour" telemetry/telecommand use. Equipment able to use these slots has been near invisible locally,& an additional appeal of these Sure modules is that they apparently can be programmed to cover ~477 MHz frequencies. Although low power, the Sure/APC modules may thereby allow some use of these otherwise neglected UHF slots at 476.950 MHz (Ch.22) & 476.975 MHz (Ch.23).

Stan.
 
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D

Dippy

Moderator
Yes, after some mixed results with that Microcgip RF chip I too will be experimenting with GFSK at some stage. I had a quick look at the technical aspects and it seems attractive.
As I'll be testing this at the nitty-gritty end I'll have an experiment with pros and cons.
Hopefully, I'll get a chance to do some Real World tests of 'deafness' versus data rates too.

For anyone entertaining the notion of the MRF chip, please note that after a couple of days of full-time faffing and talking to michip tech, there are (confirmed) errors in data sheet and oddities in parts of the C snippets from Mchip. Personally, I wouldn't bother with it. Pity, the spec and claims were good ;) Obv. it's not impossible but I've run out of time on it.

Stan, I didn't eat Al.
 
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manuka

Senior Member
FYI - the good Dr_A continues to be impressed with these SURE RMB-CM12111.
"They are working brilliantly. File transfers so much faster at 9600 (Yishi can do 9600 but it is factory set and I went for 1200 Yishi so I am stuck with that). The Sure ones can be changed as needed. The huge buffer means no issues at all with file transfers etc."
Click to expand...
In a nutshell they're powerful (a legal 20mW), sensitive (~ -117dBm at 1200 bps),wide frequency settings, narrow bandwidth(GFSK),versatile (TTL/RS232 etc), configurable,reasonably priced,look widely available ( Ebay stores etc),accept standard antenna and are very small. We've conducting further tests (ahem- in conjunction with a certain Australian supplier) & will keep you posted.
 

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moxhamj

New Member
A few more things with the Sure modules - they are TTL and RS232 and RS485. RS485 is an interesting standard in that you can have a multidrop bus and because the signals are complementary the range is a lot further than RS232. I found a circuit in this month's Circuit Cellar that has a RS485 router which is a concept I'd like to explore further and/or link with RF modules.

Plus with the Sure modules the Tx power can be changed so you can save power by going for the minimum power that does the job.
 
S

SAborn

Senior Member
Can the Sure ones be driven by only one pin on the picaxe like the cheap 433mhz ones.
Or what is required to operate these from the picaxe 2...3? pins.
 
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moxhamj

New Member
Yes you can drive them with one pin. They are 'transceivers' so can transmit and receive. So you could use one just to transmit and that would work fine too. It depends whether you need data going both ways or just one way and that depends on the application.
 
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moxhamj

New Member
You mean the double post? I could claim too much coffee, but no, it was the forum software hanging then I had to open a new browser window, log in and copy and paste (lucky that still works when the forum hangs) and finally about 20 mins later the other post went through. Maybe the IP packets went via Mars or something?

Just a thought also comparing these with the cheap 'raw rf' modules, these transceiver ones don't need the UUUU at the beginning. They behave just like a wired connection - what goes in comes out the other end. Much more reliable and easy to work with. It is pretty cool to transfer a 50kilobyte file and not have one byte with an error.
 
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G

Goeytex

Senior Member
The Sure modules look quite attractive both in performance and price and they include an antenna. I just ordered a set to test.
 
J

john2051

New Member
re: 434MHz

Hi
We seem to have problems here in lincoln, being in close proximity to one or two amateur radio repeaters operating near the same frequency. They can make these modules useless for anything but a very short distance. As a licenced amateur I can use 400Watts in this band, so its bound to de-sensitize trf receiver modules serveral mhz away.
The higher the frequency the better i think, because you can have a high gain antenna that is physically small.
kind regards

john
 
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Goeytex

Senior Member
Yep, that could be an issue in some areas. I have a HAM neighbor and he sometimes messes stuff up around here with his old noisy junk.

I will also be evaluating This Stuff from Radiotronics. A bit pricey but looks nearly goof proof if care is taken.

Goey
 
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SAborn

Senior Member
While searching ebay a few days ago for transceivers i came across some 2.4 gig modules for around the same price as the sure ones. (little dearer)

Has anyone used the 2.4 gig tranceivers and with what results.

I would like to get a reliabale distance of at least 100m or better from a shed to a house in most situations for data logging.

I to have been tempted to try the sure ones and am watching this thread with keen interest.
 
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manuka

Senior Member
It's "horses for courses", and no single wireless data approach generally suits everyone for all applications. Diverse tradeoffs arise with regulations,power ratings,setup and connectivity hassles,antenna sizes,ranges, interference,data rates,path loss & of course BUDGETS. You can not assume higher 2.4GHz freqs. will be interference free either, since numerous devices now use that slot. Microwave ovens may be well shielded, but toys, controllers, WiFi, Bluetooth, ZigBee & even cordless phones abound - the latter being especially a pain when used near APs (WiFi Access Points).

However perhaps THE major issue at increasingly higher frequencies relates to propagation path obstructions. It's well known that almost anything metallic/watery in the way will block weak 2.4GHz signals-even your own "watery" body! 1mW 2.4GHz ZigBee links usually conk out with the slightest obstruction (including animals/birds/humans) at well <100m, but if the path is totally clear dish beaming may extend such ZigBee ranges to km LOS (line of sight).

Few PICAXE applications need (or can work at!) anything like the 100s kHz- MHz data rates that 2.4 GHz technologies can support anyway, & in contrast they usually DO need the ability to punch thru' vegetation and modest building/terrain/watery obstructions. Hence (IMHO),as lower frequencies have more propagation punch,on this aspect alone sensitive 1200-9600 bps 433 MHz devices may be superior to 2.4 GHz for PICAXE applications.
 
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S

SAborn

Senior Member
manuka

Thanks for that it was what i had thought but it is nice to have a second opion without spending the dollars to find out the hard way.

Here is a link to another supplier for RF modules that might be of interest to some in this discussion
http://www.futurlec.com.au/RadioModules.jsp

I undrestand they are slow at shipment but some of their pricing looks good.
 
G

Goeytex

Senior Member
As was suggested already, the application and conditions can determine what
device is suitable. One size does not necessarily fit all.

If the signal must penetrate obstacles such as walls, trees, etc then the lower
frequency are a good choice. However, if there is clear line of sight then
devices in the 900 Mhz or 2.x Gighz range might be a good choice.

Hmm. That 500mw transceiver from Futurlec (Hope) might get out there
a ways .....

Goey
 
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manuka

Senior Member
Of interest to Australians? Nth Sydney firm Little Bird Electronics (LBE) (who are SparkFun resellers), are taking on the nifty Sure 433 transceivers. Their GST inclusive prices of Aust $45 a pair (includes antenna) look attractive.

Disclosure: Although I may feature them in a "Silicon Chip" article, I am in no way associated with LBE or Sure Electronics,and merely consider these modules a significant and cost effective enhancement to existing 433 MHz data transceiver offerings.
 
R

RavenKallen

New Member
I have used those modules on sparkfun. They do work pretty well for being so cheap. It is possible to multiplex them if you only have one transmiting at a time. The master picaxe should control all of the others, by sending a unique id number to all of the slave devices. If a slave device "sees" its id then it will accept the rest of the message. The master will then clear the air and give the slave enough time to finish transmitting its message. Then the master could take that message and do some process with it or take it and re-TX it to another slave device, which in turn could do the same. The thing is you can only have one transmitter TXing at one time, that is why the master device controls all TXing and RXing. SO yes, it is feasible.
 
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moxhamj

New Member
I think all modules apply in this thread.

I'm pondering at the moment what it would take to build a 'router' that can handle translation from one format to another. eg the cheap 'raw rf' to the more expensive ones that have microprocessors on board (eg Hope/Sure). Then it wouldn't be so important which module is the best, and a wireless network could be updated as more modules become available.

I think the key here is data packets and I'm doing some experiments at the moment. Stay tuned!
 
H

hisem

New Member
manuka said:
FYI - the good Dr_A continues to be impressed with these SURE RMB-CM12111. In a nutshell they're powerful (a legal 20mW), sensitive (~ -117dBm at 1200 bps),wide frequency settings, narrow bandwidth(GFSK),versatile (TTL/RS232 etc), configurable,reasonably priced,look widely available ( Ebay stores etc),accept standard antenna and are very small. We've conducting further tests (ahem- in conjunction with a certain Australian supplier) & will keep you posted.
Click to expand...
I have been looking at these Sure transceivers for awhile. After reading all these posts about them I purchased a pair, they should be on the way. Could anyone point me in the direction of information on how I will use them with the picaxe? The datasheet scared me a bit as it seemed I needed a "Windoze" computer to program them first before I could use them. I don't want to "have to" use their "RF Magic" software. Can I not set the settings some other way, preferably by the picaxe? Any help or pointer in the direction of tutorials and explanations would be MUCH apreciated :)
This will be my first attempt at using transceivers.
Thanks.
 
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moxhamj

New Member
The RF magic software is fairly simple to use. The default is RS485 and you might want to change that to RS232, but then again, talking directly to a picaxe, you would just use the TTL pins. RS232 could still be useful linking a module to a PC.

Next issue is the baud rate, and while 9600 is nice, it might be a bit fast for some of the picaxes. So you might have to drop that.

Those are the only two settings that need changing. The only other complexity is setting up the module on a breadboard in order to program it. Manuka has a couple of modules so stay tuned for some photos soon...
 
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